In wireless telecommunications, radio signals propagate from a sender to a receiver on multiple paths due to, for example, reflection from buildings or other objects. Multipath components are the original signal and echoes of the original transmitted signal traveling through different paths. Multipath components arrive at the receiver each with a different magnitude, phase and time-of-arrival, causing constructive and destructive interference and phase shifting of the signal at the receiver side.
A rake receiver is a radio receiver designed to counter the effects of multipath fading. It contains sub-receivers with correlators, each assigned to a different multipath component. Each component contains the original information and if the magnitude and phase of each component is computed at the receiver, all components can be added coherently to improve the information reliability. Each correlator independently decodes a single multipath component, and the contributions of the correlators are combined.
For cellular telephone networks, several standards for enabling the radio-based communication link between a cellular phone or other mobile station or cellular communication device and a cellular base station are available, employing different channel access methods.
Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA employs a spread-spectrum technology and a coding scheme, i.e., the coded baseband modulated signal has a higher data bandwidth than the transmitted data, wherein each channel, i.e. each signal, for transmitting data from a sender to a receiver is assigned a code to allow multiple channels to be multiplexed over the same frequency band or physical channel.
The W-CDMA (Wideband Code Division Multiple Access) standard partly makes use of the CDMA channel access method and is the basis of the FOMA (Freedom of Mobile Multimedia Access) third generation telecommunications service in Japan. UMTS (Universal Mobile Telecommunications System) is a third generation mobile cellular technology for networks based on the GSM standard and widely used in Europe. UMTS is a component of the International Telecommunications Union IMT-2000 standard set and also uses the W-CDMA radio access technology standard.
CDMA2000, or IMT Multi Carrier (IMT MC), is a family of third generation (3G) mobile technology standards, for example used in the United States of America. It also partly employs the CDMA channel access method for communication between cellular phones and cellular base stations.
W-CDMA and CDMA2000 are both based on descrambling and de-spreading the received channel data, utilizing a rake receiver, but nonetheless receivers are usually built as separate devices, because several components are usually implemented differently to comply with the respective standard requirements. For example, W-CDMA and CDMA2000 use different scrambling codes and different spreading codes. W-CDMA contains the channel processing steps of descrambling and de-spreading serially one after another; while in CDMA2000 systems, they may be performed at different stages. W-CDMA has complex-valued channels, while CDMA2000 has both complex and non-complex channels. W-CDMA has asynchronous users, while CDMA2000, at least partly, has synchronous users. W-CDMA has a slot length different from that of CDMA2000, wherein in the CDMA2000 standard family two different slot lengths are used for EvDO (Evolution-Data Optimized) and 1×RTT (1 times Radio Transmission Technology). For W-CDMA and CDMA2000, the modulo of the code is different. A W-CDMA frame consists of 150 sub-slots, whereas the frame length in CDMA2000 is 128 for EvDO, short and long, and 128 for 1×RTT, short only, while long is a rolling code which is a function of previous execution. Also, the chip-rate in W-CDMA is 3.84 Mcps while in CDMA2000 it is either 1.28 Mcps or 3.84 Mcps (megachips per second).